Recent years have seen intensified interest in the use of aqueous coating materials from the viewpoint of environmental conservation, reduced usage of resources and energy savings, and aqueous coating materials are also being developed for “metallic paints” that are applied as finish coatings on automobile bodies and the like.
A problem commonly associated with aqueous coating materials is the difficulty of achieving both low viscosity through atomization by spraying or the like, which is preferable for producing coated surfaces with an excellent finished appearance, and high viscosity which is preferred for avoiding post-coating sagging or preventing sedimentation of pigments during storage.
It has been attempted to circumvent this problem with coating material designs that impart pseudoplastic viscosity to aqueous-based coating compositions.
Pseudoplastic viscosity is a phenomenon whereby viscosity is reduced with increasing shear rate, and it is well known in the field of coating materials that a coating composition exhibiting pseudoplastic viscosity is able to exhibit a sufficiently low viscosity for atomization under high shear rates such as during spraying and a sufficiently high viscosity so as not to sagging under low shear rates such as during coating, so that both coated surface smoothness and sagging resistance can be achieved.
Compositions containing polymer fine particles and acrylic copolymer aqueous solutions are well known as means of imparting pseudoplastic viscosity to aqueous coating compositions (for example, Japanese Examined Patent Publication HEI No. 2-32947: Patent document 1).
As another example of a composition containing polymer fine particles and an acrylic copolymer aqueous solution, Japanese Unexamined Patent Publication HEI No. 7-53913 (Patent document 2) describes an aqueous coating composition containing an aqueous dispersion of carboxyl group-containing acrylic resin particles and a film-forming polymer obtained by neutralizing at least some of the acidic groups in a copolymer of an amide group-containing ethylenic unsaturated monomer, an acidic group-containing ethylenic unsaturated monomer and a hydroxyl-containing ethylenic unsaturated monomer, as a metallic based coating material with low sagging and excellent coating manageability which is also able to form coated films with improved outer appearance.
Also, as a base coat coating material capable of forming coated films with excellent water resistance and adhesion, Japanese Unexamined Patent Publication HEI No. 10-306236 (Patent document 3) describes an aqueous coating composition comprising an acrylic resin emulsion and/or urethane resin emulsion, and a water-soluble acrylic resin obtained by using a neutralizer to neutralize an acrylic copolymer that is produced by copolymerization of an alkylene oxide group-containing (meth)acrylate, a cyclic saturated hydrocarbon group-containing (meth)acrylate, and another unsaturated monomer.
The compositions described in Patent documents 1-3 employ water-soluble resins, obtained using neutralizers for neutralization of the acidic groups of acidic group-containing resins, as coated film-forming components for combination with polymer fine particles, from the standpoint of ensuring solubility in aqueous media, achieving satisfactory storage stability as coating materials and exhibiting sufficient affinity for pigments, and such water-soluble resins are now commonly employed in the field.